A typical positioning application to which the present invention can be applied is the movement of a magnetic head (transducer) between information bearing concentric tracks in a magnetic disk file of the so called "sector-servo" type. In such a file, servo position reference information is recorded in a plurality of spaced sectors interleaved between larger sectors on which the data is recorded. An advantage of such a system is that the servo information is contiguous with the recorded data so that the data tracks of interest can be followed with greater accuracy and thus packed closer together. The servo sectors must be sampled at times defined by clock signals to enable a position error to be generated for track following purposes.
Such a type of disk file is described in U.S. Pat. No. 3,185,972 (Sippel) which fully describes the track following operation. Although the Sippel patent refers to a coarse positioning system for moving the head between tracks, it does not describe this coarse positioning system in detail.
The time taken to move a head between tracks in a disk file is known as the "access" time and is one of the most important performance characteristics of a file. To minimize the access time for a file of given mechanical configuration and actuator performance requires an access motion control system which will control the velocity of the head in time optimal fashion and which will bring the head accurately to rest on the desired track.
The access motion is, therefore, necessarily of wide bandwidth and the access control system is subject to the stability and error constraints of such systems. Conventionally, these wide band requirements have necessitated the use of a continuous position reference source such as a separate servo disk. In such a system, near time-optimal access motion has been accomplished by means of a derived continuous distance-to-go signal acting on a reference velocity curve generator which, via a high gain closed loop, forces the actual velocity of the head to follow a timeoptimal reference velocity profile from the curve generator.
This conventional approach is not available with a sector servo system since direct head position and velocity information is only available at servo sector sampling times. It is thus difficult to reconcile the use of servo sectors in a disk file with low access times.
Various access control schemes for sector servo files have been proposed in the prior art. One of these, described in U.S. Pat. No. 4,103,314 "Motion Control System" (inventor W. J. P. Case), and assigned to the same assignee as the present applications, is an access control system in which the actuator is energized to cause the head to follow a constant velocity portion of a desired velocity profile. The constant velocity is such that the passage of the head over track centers is synchronized with the timing of the servo sectors. The normal servo sector position error signal, as also generated during track following, may thus be used during the access motion to keep the velocity constant. During brief initial acceleration and final deceleration stages of the motion the full power supply voltage is applied to the actuator under open loop conditions. The motion of the head during an access does not approach time optimal motion since it is at constant velocity over all but a few tracks. The constant velocity is low as the head only traverses one track per two sector periods and must be synchronized with the sector frequency. Furthermore, only in a low velocity system is it possible to effect the final deceleration under open loop conditions without significant final position error.
Another access control system for a sector servo file is described in U.K. Pat. No. 1,527,950, Cardot et al. This patent employs the so-called "bang-bang" technique of controlling head motion in which the maximum available power is used for both acceleration and deceleration. The system is switched between full forward and full reverse power at a point which is calculated from the initial and target track addresses. The servo sectors are coded with track address information which is read by the head during the access motion and used to determine when the power is to be reversed. Although allowing the highest possible speeds to be attained during access motion, the described system does not employ any form of closed loop control during acceleration and deceleration. The position of the head when it comes to rest is thus unknown until a comparison can be made of the actual address of the track over which the head is most nearly situated with the target address. There is provision for a further shift of the head if the two addresses are not equal. Such shifts would add to the average access time.
More recently in the prior art a pending U.S. patent application Ser. No. 811,350 ("Positioning System employing Feedforward and Feedback Control"; Inventor: R. K. Oswald), assigned to the same assignee as the present application, has described an access control system for a disk file which is relevant to the sector servo problem.
The Oswald application is basically concerned with access control systems for use with files having continuous servo position information. It proposes that a combination of feedforward and feedback control can enable high performance accesses even with low bandwidth fed back position information. A feedforward signal, which is a prediction of required drive current in a nominal system, is applied to the head actuator to cause it to execute approximately a high performance (high bandwidth) access. At the same time, low bandwidth head position or velocity information is fed back and compared with a stored velocity profile to correct small deviations between the nominal and actual systems. In one embodiment, transducer incremental position signals are employed as the fed back quantity. These are compared in phase with a "model" incremental position signal derived from a stored acceleration velocity profile and a stored deceleration velocity profile which the system is effectively forced to follow. The Oswald application suggests that this technique is applicable to the problem of sector servo access control.
To complete the discussion of the prior art, reference is made to another U.S. Pat. No. 4,133,011 ("Sampled Data Positioning System Employing a Model of the Physical System for Time Optimal Control"; Inventor: F. Kurzweil Jr.), assigned to the same assignee as the present application. This application describes a sampled data positioning system, specifically the access control system of a sector servo disk file, which employs a model of the physical system to overcome the low bandwidth constraints of fed back position and velocity samples. The model system is an approximate analog of the entire physical system and provides continuous (i.e. high bandwidth) pseudo position and velocity signals. These are applied to an access controller which generates an error signal between a desired reference velocity profile and the pseudo velocity signal exactly in the manner of the conventional system with continuous dedicated servo reference information. The error signal is fed back to the model system but is also applied to control the physical system. The model velocity and position outputs are reset at sampling times to the values of the sampled velocity and position in the physical system. The servo sectors described in the Kurzweil application include track addresses as well as track following position information and the model and physical position signals are absolute (relative to the target address) rather than incremental (relative to the nearest track center).